Even if they DO offer paint services for the Diffusor alone - trouble is, it's actually part of the bumper.
They don't like to prepaint due to possible paint mismatch - if you have the bumper painted by local shop and they custom mix the paint to match the rest of your car - then the diffusor painted under standard paint code will not match the bumper.
Whether or not you will be able to notice this due to diffusor being located somewhat low on car (shadows and all)
Personally, I would much rather have a high quality local shop paint my parts to ensure proper match of all. The shop I have all my work done charges $350 to prep/paint/clear an entire rear bumper with Lifetime Warranty. Pretty sure you're not gonna find a quality job for less than that.
Plus, with a local shop, if by chance there IS any paint mismatch - they can re-do for you immediately and there would be no charge. That's not gonna happen if you have it painted out of state before it arrives...
Believe me, if the bumper paint is even a little off - you WILL notice it very easily. I had my wife's front bumper repainted in a Metallic Blue by different shop than I get BMW work done - apparently there were a couple different paint mixes for the same paint code - they chose the wrong one and I could tell immediately, even at dusk. They repainted at their cost and 2nd time match was dead on.